Merged characters hand to the arm and got a weird choppy result
Normals was inverted,
you can flip normals of selected faces while in Edit mode by:

3D Viewport Header -> Mesh -> Faces -> Flip Normals

If you want to quickly fix all the normals in your mesh, while in Edit mode you can use Recalculate Normals:


3D Viewport Header -> Mesh -> Normals -> Recalculate Outside (CTRL + N)
3D Viewport Header -> Mesh -> Normals -> Recalculate Inside (SHIFT + CTRL + N)


  Why are some faces in my mesh darker?



Thanks to Mr Zak &amp; Thom Blair III